1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device including a circuit formed of a thin film transistor (hereinafter referred to as a TFT) using a crystalline semiconductor film formed on a substrate having an insulating surface. Particularly, the present invention relates to a liquid crystal display device in which a pixel portion and a driving circuit provided at its periphery are provided on the same substrate and an electrical instrument (called electronic apparatus as well) using the liquid crystal display device as a display portion.
2. Description of the Related Art
With the rapid development of an information-oriented society, information appliances including a personal computer (PC) rapidly become popular for not only a business enterprise but also an individual. From the viewpoint of space saving of portable information equipment or a display of the PC, a liquid crystal display device (liquid crystal display) is regarded as being promising from the early days. However, there has been a problem that a manufacturing process of the liquid crystal display device is complicated and its yield is low, and accordingly, manufacturing costs are high.
Besides, in recent years, from a problem of a field effect mobility, technical development of a thin film transistor (hereinafter referred to as a TFT) using a polycrystalline semiconductor film, for example, silicon as semiconductor (hereinafter referred to as a polysilicon film or a crystalline silicon film), which has a polycrystalline state obtained by crystallizing a noncrystalline semiconductor film (hereinafter referred to as an amorphous silicon film) formed on an insulating surface provided on a substrate (for example, a glass substrate, a quartz substrate, a stainless steel substrate, etc.), has been rapidly advanced. Especially, a polycrystalline silicon film prepared by carrying out a heating treatment for crystallization at a low temperature (600xc2x0 C. or lower) is called a low temperature polysilicon film.
In recent years, researches have been carried out to construct a semiconductor circuit by forming TFTs on a lass substrate or the like. As an electric device such a semiconductor circuit, an electro-optic device such as an active matrix type liquid crystal display device is typical.
The active matrix type liquid crystal display device is a monolithic display device in which a pixel matrix circuit and a driver circuit are provided over the same substrate. Further, the development of a system-on-panel having a built-in logic circuit, such as a memory circuit or a clock generating circuit, has also been advanced.
Since such a driver circuit or a logic circuit requires performing a high speed operation, it is unsuitable to use a noncrystalline silicon film (amorphous silicon film) as an active layer. Thus, in the present circumstances, a TFT including, a crystalline silicon film (polysilicon film) as an active layer has become mainstream.
Then, research and development has been actively carried out as to a process, a so-called low temperature process, for forming a large area crystalline silicon film on a substrate having low heat resistance as compared with a quartz substrate, such as a glass substrate.
As a method of preparing a low temperature polysilicon film, a laser annealing, method, an ion doping method, or the like is mainly used. As a method of obtaining a high quality low temperature polysilicon film, a technique using a metal element as a catalytic element for facilitating crystallization is disclosed in Japanese Patent Application Laid-open No. Hei 7-183540, etc. As the metal element, nickel (Ni), palladium (Pd), lead (Pb), tin (Sn) or the like is used. The catalytic element is added to a semiconductor (silicon) film by a method, such as a solution coating method, a sputtering method, an ion implantation method, an evaporation method, or a plasma treatment method, and a heating treatment for crystallization is carried out. However, there has been a problem that although such a treatment can be carried out at a low temperature, a treatment time is long.
The present inventors et al. disclose a technique for obtaining a crystalline silicon film on a glass substrate in Japanese Patent Application Laid-open No. Hei 7-130652. In the technique of the invention, a catalytic element for facilitating crystallization is added to an amorphous silicon film, and a heating treatment is carried out to crystallize the amorphous silicon film.
By this crystallization technique, it became possible to lower the crystallization temperature of the amorphous silicon film by 50 to 100xc2x0 C., and to shorten a time required for crystallization by a factor of ⅕ to {fraction (1/10)}. As a result, it became possible to form a large area crystallized silicon film even on a glass substrate having low heat resistance. It is experimentally confirmed that the crystalline silicon film obtained by such a low temperature process has excellent crystallinity.
Besides, an environmental problem becomes more serious, and it is emergently required to take energy-saving measures with respect to electric appliances at worldwide level. Then, in order to achieve such an object as improvement of efficiency of a manufacturing process for mass production of liquid crystal cells or reduction of manufacturing costs, enlargement of a substrate in the manufacturing process is required, and technical development for obtaining a plurality of TFT substrates from a large glass substrate has been advanced.
Incidentally, in the present specification, the liquid crystal cell indicates a display device in a state where a liquid crystal is interposed between a substrate on which pixel TFTs are formed and a counter substrate.
The present applicant discloses, in Japanese Patent Application Laid-open No. Hei 7-130652, a method of fabricate a crystalline semiconductor film having high crystallinity by adding a metal element (hereinafter referred to as a catalytic element) having a function of facilitating crystallization to an amorphous semiconductor film in a crystallization step and by carrying out a heating treatment.
However, the method of the above invention is a heating treatment using a furnace, and it takes a rather long time, for example, 1 to 14 hours to carry out the heating treatment and to form the crystalline semiconductor film.
In the manufacturing process for actually mass-producing semiconductor devices, shortening of a treatment time is an important problem. Besides, as another technique for improving the efficiency of the manufacturing process, the establishment of a technique of manufacturing a plurality of liquid crystal cells, for example, six liquid crystal cells each having a size of 12.1 inches from one large glass substrate, for example, a substrate of 550 mmxc3x97650 mm has also been advanced. In future, a technique and a manufacturing, apparatus for manufacturing more liquid crystal cells from a larger glass substrate are required to be introduced. With the enlargement of another member (glass substrate) before treatment, an apparatus used for the manufacturing process is naturally required to be enlarged, and a furnace for carrying out the heating treatment has a problem on the enlargement of an installation area, and needs energy for uniformly and sufficiently heating the large furnace for treating the large substrate as set forth above, and there has been a problem that the energy becomes enormous electric power consumption.
Then, in view of the efficiency of manufacture and the improvement of productivity, it is conceivable that an RTA (Rapid Thermal Anneal) method is suitable as a heating method. However, the RTA method is a method in which a heating treatment of a high temperature and a short time is carried out for the purpose of suppressing the diffusion of an impurity in a semiconductor layer, and in a heating treatment step of a semiconductor film requiring the diffusion of an element, such as a crystallization step using a catalytic element or a gettering step, there is a possibility that a glass substrate is distorted before a desired effect is obtained. For example, it is confirmed that in the gettering step in a furnace, the glass substrate is curved and deformed by its own weight by merely carrying out a treatment at 800xc2x0 C. for 60 seconds.
An object of the present invention is therefore to solve the above problems and to provide a method of efficiently fabricating an excellent crystalline semiconductor film on a large glass substrate in order to enable mass production using a large substrate.
It is known that when a heating treatment is carried out in a high temperature state exceeding 600xc2x0 C., high speed growth of an oxidized semiconductor film due to a catalytic element occurs, and breakdown of a formed semiconductor component occurs. Further, it is known that when a heating treatment is carried out in a high temperature state exceeding 900xc2x0 C., an oxidized semiconductor film grows at high speed even in a region where a catalytic element is not contained.
Another object of the present invention is therefore to shorten a time required for a process by controlling a light source to irradiate a pulsed light in a heating treatment for crystallization.
Still another object of the present invention is to reduce hydrogen in a film to improve crystallinity by carrying out a heating treatment for crystallization in a reduced pressure atmosphere. Besides, still another object of the present invention is to reduce an oxygen concentration in an atmosphere and to suppress formation of an oxide of a catalytic element for facilitating crystallization by carrying out a heating treatment in a reduced pressure atmosphere. Besides, still another object of the present invention is to facilitate crystallization and to shorten a crystallization time by carrying out a heating treatment for crystallization under vacuum. Besides, still another object of the present invention is to reduce hydrogen in a film and to improve crystallinity by carrying out a heating treatment for crystallization under vacuum. Besides, still another object of the present invention is to reduce an oxygen concentration in an atmosphere and to suppress formation of an oxide of a catalytic element for facilitating crystallization by carrying out a heating treatment under vacuum.
If a catalytic element at a high concentration remains in a semiconductor film after an excellent crystalline semiconductor film is fabricated on a glass substrate by a low temperature process, the catalytic element forms a deep energy level in the semiconductor film (silicon film) and traps a carrier and is recombined. Accordingly, if a TFT is formed by using a thus obtained crystalline silicon film, it is expected that the electrical characteristics and reliability of the TFT is badly affected, which is another problem.
It is confirmed that the catalytic element remaining in the crystalline semiconductor film is segregated irregularly, especially in a crystal grain boundary concentratedly, and if this segregation exists in a region which becomes a component in the semiconductor film (especially in a channel formation region and a connection portion between the channel formation region and a source region or a drain region), it is considered that the segregation becomes an escape route (leak path) of a weak current, and causes an abrupt increase in an off current (current when a TFT is in an off state).
Yet another object of the present invention is therefore to provide a method in which a gettering step for quickly reducing the concentration of a catalytic element remaining in a crystalline semiconductor film after a crystallization step using the catalytic element is also carried out by a low temperature process.
It becomes possible to improve the throughput of the heating treatment by using an RTA apparatus as set forth above. Besides, when a light source is made to irradiate in a pulsed manner, a treatment temperature can be lowered before heat is conducted to glass, so that a heating treatment of a semiconductor film formed on a glass substrate becomes possible.
Further, heat transfer by lighting of the light source controlled in a pulsed manner is controlled through a temperature sensor, and cooling means for preventing transfer of heat exceeding the glass transition temperature to the glass substrate is used in accordance with this control. Since heating and cooling are carried out at the same time, it is possible to prevent the temperature from exceeding the glass transition temperature during the heating treatment or to shorten a time in which the temperature exceeds it. Besides, by repeating this heating treatment, even in a period in which such a temperature that a catalytic element for facilitating crystallization of a semiconductor film is diffused in the semiconductor is held, the glass substrate is not deformed, and the heating treatment of the semiconductor film for crystallization of the semiconductor film and for gettering of the catalytic element can be efficiently carried out in a relatively short time. This method is called a xe2x80x9cPlural Pulse Thermal Annealingxe2x80x9d (hereinafter referred to as PPTA) in the present specification.
The PPTA (Plural Pulse Thermal Anneal) apparatus uses a heating method capable of performing rapid heating and rapid cooling in which a light source is made to emit light in a pulsed manner to irradiate so that only a semiconductor film is instantaneously heated and the heating can be stopped before the heat is conducted to a glass substrate. Thus, the glass substrate is not deformed or damaged by heat. Further, the heat transfer by lighting of the light source controlled in the pulsed manner is controlled with a temperature sensor, and cooling means for preventing transfer of heat exceeding the glass transition temperature to the glass substrate is used in accordance with this control. Besides, by repeating this heating treatment, even in a period in which such a temperature that a catalytic element for facilitating crystallization of the semiconductor film is diffused in the semiconductor is held, the glass substrate is not deformed, and the heating, treatment of the semiconductor film for crystallization of the semiconductor film and for gettering, of the catalytic element can be efficiently carried out in a relatively short time.
Incidentally, FIG. 20 shows an example of the PPTA (Plural Pulse Thermal Anneal) apparatus. In FIG. 20, a first heat treatment chamber 751, a second heat treatment chamber 752, and a third heat treatment chamber 753 are connected to the circumference of a first transport chamber 750 through gates 772d to 772f. The structure of these heat treatment chambers is the same as that of FIG. 1. A refrigerant is introduced into the respective heat treatment chambers through flow control means 767 from a cylinder 766. Exhausting means for reducing the pressure in the process chamber is constructed by turbo molecular pumps 768a to 768c and dry pumps 769a to 769c. Besides, there are provided circulators 771a to 771c for circulating the refrigerant and purifiers 770a to 770c for purifying the refrigerant. Although not shown, turning on and off of a light source and supply of the refrigerant are controlled by a computer. In the treatment chambers are equipped light sources 762a-762c and substrate stages 763a-763c, respectively.
The second transport chamber 754 is provided with transport means 760, which transports a substrate to be treated to the first treatment chamber 750, a surface treatment chamber 755, and a cooling chamber 756. The surface treatment chamber 755 is provided with a spinner 764. The cooling chamber 756 is provided with a substrate stage 765. In the structure of a load chamber 757 and an unload chamber 758, the movement of the substrate to be treated is made by transport means 761. Note, reference numeral 759 indicates transport means.
The present invention is a method of fabricating a semiconductor device using the heating treatment apparatus as described above, and is characterized by comprising a first step of adding a catalytic element for facilitating, crystallization to an amorphous semiconductor film formed on an insulating surface, and a second step of forming a crystalline semiconductor film by controlling a light source to irradiate a pulsed light to the amorphous semiconductor film to crystallize it.
Besides, the present invention is a method of fabricating a semiconductor device using the heating treatment apparatus as described above, and is characterized by comprising a first step of adding a catalytic element for facilitating crystallization to an amorphous semiconductor film formed on an insulating surface, and a second step of forming a crystalline semiconductor film by controlling a light source to irradiate a pulsed light to the amorphous semiconductor film to crystallize it, wherein a light emitting time of the light source is 1 to 60 seconds.
The above invention is characterized by comprising a step of improving crystallinity by irradiating a laser light to the crystalline semiconductor film after the second step.
The above invention is characterized in that in the second step, the inside of the treatment chamber has a reduced pressure atmosphere.
The above invention is characterized in that in the second step, an atmosphere in the treatment chamber contains oxygen at a concentration of 5 ppm or less.
Besides, the present invention is a method of fabricating a semiconductor device by using the heating treatment apparatus as described above and is characterized by comprising a first step of adding an impurity element to a crystalline semiconductor film formed by adding a catalytic element to an amorphous semiconductor film and carrying out a heating treatment, and a second step of irradiating a pulsed light to the crystalline semiconductor film added with the impurity element by controlling a light source, wherein gettering of the catalytic element is carried out by the light irradiation treatment.
Besides, the present invention is a method of fabricating a semiconductor device by using the heating treatment apparatus as described above and is characterized by comprising a first step of adding an impurity element to a crystalline semiconductor film formed by adding a catalytic element to an amorphous semiconductor film and carrying out a heating treatment, and a second step of gettering the catalytic element by controlling a light source to irradiate a pulsed light to the crystalline semiconductor film added with the impurity element, wherein a light emitting time of the light source is 1 to 40 seconds.
The above invention is characterized in that the impurity element is an element belonging to group 15 of the periodic table.
The above invention is characterized in that the impurity element is an element belonging to group 15 of the periodic table and an element belonging to group 13 of the periodic table.
The above invention is characterized in that the impurity element is an element belonging to group 18 of the periodic cable, an element belonging to group 15 of the periodic table, and an element belonging to group 13 of the periodic table.
The above invention is characterized in that the concentration of the impurity element belonging to the group 13 is {fraction (1/100)} to 100 times as high as the concentration of the impurity element belonging to the group 15.
Besides, the present invention is a method of fabricating, a semiconductor device by using the heating treatment apparatus as described above and is characterized by comprising a first step of forming an amorphous semiconductor film on a crystalline semiconductor film formed by adding a catalytic element to an amorphous semiconductor film and carrying out a heating treatment, and adding an impurity element to the amorphous semiconductor film, and a second step of gettering the catalytic element by controlling a light source to irradiate a pulsed light to the crystalline semiconductor film.
The above invention is characterized in that the impurity element is an element belonging to group 18 of the periodic table.
The above invention is characterized in that the impurity element is an element belonging to group 18 of the periodic table, an element belonging to group 15 of the periodic table, and an element belonging to group 13 of the periodic table.
The above invention is characterized in that in the second step, the inside of a treatment chamber is exhausted and its pressure is 26.6 Pa or less.
The above invention is characterized in that in the second step, an atmosphere in a treatment chamber, especially in the vicinity of the crystalline semiconductor film contains oxygen at a concentration of 2 ppm or less.
The above invention is characterized in that the impurity element belonging to the group 15 of the periodic table is an element selected from N, P, As, Sb and Bi.
The above invention is characterized in that the impurity element belonging to the group 13 of the periodic table is an element selected from B, Al, Ga, In and Tl.
The above invention is characterized in that the impurity element belonging to the group 18 of the periodic table is an element selected from Ar, Kr and Xc.
The above invention is characterized in that in the second step, a continuous holding time of a temperature exceeding a glass strain point is 20 seconds or less.
The above invention is characterized in that a holding time of highest intensity of the light source in the second step is 1 to 5 seconds.
The above invention is characterized in that in the second step, cooling using a nitrogen gas, an inert gas or a liquid as a refrigerant is carried out simultaneously.
The above invention is characterized in that in the second step, a vicinity of the crystalline semiconductor film is in a nitrogen (N2) atmosphere, an inert gas atmosphere, a hydrogen (H2) atmosphere, or a reducing gas atmosphere.
The above invention is characterized in that the light source is a light source for emitting infrared light or ultraviolet light.
The above invention is characterized in that a halogen lamp, a metal halide lamp, a xenon arc lamp, or a reduced pressure mercury lamp is used as the light source.
The above invention is characterized in that the light source irradiates an upper side of the substrate, a lower side of the substrate, or the lower side and the upper side of the substrate.
The above invention is characterized in that the catalytic element is one or plural kinds of elements selected from Ni, Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au.
Besides, the present invention is a method of fabricating a semiconductor device using the heating treatment apparatus as described above and is characterized by comprising a first step of forming an amorphous semiconductor film on an insulating surface, a second step of adding a catalytic element for facilitating crystallization to a surface of the amorphous semiconductor film, a third step of forming a crystalline semiconductor film by controlling a light source to irradiate a pulsed light to the amorphous semiconductor film added with the catalytic element to crystallize the amorphous semiconductor film, a fourth step of adding an impurity element to the crystalline semiconductor film, and a fifth step of gettering the catalytic element by controlling the light source to irradiate a pulsed light to the crystalline semiconductor film added with the impurity element.
Besides, the present invention is a method of fabricating a semiconductor device using the heating treatment apparatus as described above and is characterized by comprising a first step of forming an amorphous semiconductor film on an insulating surface, a second step of forming a catalytic element inclusion region by coating a catalytic element for facilitating crystallization onto a surface of the amorphous semiconductor film, a third step of forming a crystalline semiconductor film by controlling a light source to irradiate a pulsed light to the amorphous semiconductor film coated with the catalytic element to crystallize the amorphous semiconductor film, a fourth step of adding an impurity element to the crystalline semiconductor film, a fifth step of gettering the catalytic element by controlling the light source to irradiate a pulsed light to the crystalline semiconductor film added with the impurity element, a sixth step of transforming the crystalline semiconductor film in which the catalytic element has keen (lettered in the fifth step into a semiconductor layer of a desired shape, a seventh step of forming a gate insulating film covering the semiconductor laser, an eighth step of forming a gate electrode on the gate insulating film, a ninth step of adding an n-type impurity element to the semiconductor layer, a tenth step of adding a p-type impurity element to the semiconductor layer which becomes an active layer of a subsequent p-channel TFT, and an eleventh step of activating the impurity elements added to the semiconductor layer by controlling the light source to irradiate a pulsed light.
The above invention is characterized in that the crystallizing step of the semiconductor layer and the activating step of the impurity elements added to the semiconductor layer are carried out in a reduced pressure atmosphere in which an oxygen concentration is reduced by performing exhaustion by a rotary pump and a mechanical booster pump.
The above invention is characterized in that the impurity element added in the fourth step is an impurity element belonging to group 15 of the periodic table, and the impurity element belonging to the group 15 of the periodic table is an element selected from N, P, As, Sb and Bi.
The above invention is characterized in that the impurity element added in the fourth step is an impurity element belonging to group 15 of the periodic table and an element belonging to group 13 of the periodic table, and the impurity element belonging to the group 15 of the periodic table is an element selected from N, P, As, Sb and Bi, and the element belonging to the group 13 of the periodic table is an element selected from B, Al, Ga, In and Tl.
The above invention is characterized in that the third step and the fifth step are carried out under a condition that a vicinity of the crystalline semiconductor film has a nitrogen (N2) atmosphere, an inert gas atmosphere, a hydrogen (H2) atmosphere, or a reducing gas atmosphere.
The above invention is characterized in that the catalytic element is one kind or plural kinds of elements selected from Ni, Fe, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au.
By carrying out the crystallization treatment of the semiconductor film and the movement (gettering) treatment of the catalytic element existing in the semiconductor film by using the apparatus as described above, it becomes possible to shorten a time required for the crystallizing step of the semiconductor film and a time required for the gettering step of the catalytic element added to the semiconductor film.